1. Field of the Invention
This invention relates to fiber optic transmission lines and, more particularly, to devices for coupling light energy between pairs of individual optical fibers.
2. Description of the Prior Art
The field of fiber optics has progressed in a relatively few years from laboratory curiosities and decorative pieces to present-day systems of high sophistication for optical communication and data transmission. Optical fibers--so-called "light pipes"--are specially fabricated filaments which exhibit the property of transmitting light longitudinally along a flexible axis. Various materials which are used in the fabrication of optical fibers and the particular properties thereof are described for example in the Derick et al U.S. Pat. No. 3,508,589 and in further detail in British Pat. No. 1,037,498, cited therein.
Low-loss optical couplers are important components for fiber optic distribution networks. Since networks having a large number of branching couplers are inherently powerstarved, it is important to minimize excess losses associated with these components. Furthermore, it is desirable to have couplers for which the coupling ratio can be tailored conveniently to the unique requirements of a particular system.
Several different types of couplers for multimode fibers have been reported in the prior art. A "Y" coupler can be formed by fusing the ends of three fibers; see Fujita et al, "Optical Fiber Wave Splitting Coupler", Applied Optics, Vol. 15, page 2031 (September, 1976). However, it appears that this process is difficult to control and that the resulting device is very fragile. Suzuki et al, "Concentrated-Type Directional Coupler for Optical Fibers", Applied Optics, Vol. 15, page 2032 (September, 1976) report a coupler utilizing a pair of face-to-face microprisms.
A second type of coupler has been fabricated using graded index lenses; see Minemura et al, "Two-Way Transmission Experiments Over a Single Optical Fibre at the Same Wavelength Using Micro-Optic 3 dB Couplers", Electronics Letters, Vol. 14, page 340 (May 25, 1978). The efficiency of this and the previously mentioned couplers is very dependent on the optical wavelength and the coupling ratio is variable only at a sacrifice of coupling efficiency.
Another type of coupler consists of parallel optical fibers having relatively thin optical cladding; see Drake, "Multimode Fiber-Optic Coupler with Low Insertion Loss", Applied Optics, Vol. 17, page 3248 (Oct. 15, 1978). Light is coupled between fibers by resonant energy exchange. The principal disadvantage of this coupler is that the coupling length required for an adequate coupling ratio in most applications is quite long (several centimeters).
One technique for reducing the necessary interaction length is to thermally fuse the parallel fibers, as disclosed in Kawasaki et al, "Low-Loss Access Coupler for Multimode Optical Fibre Distribution Networks", Applied Optics, Vol. 16, page 1794 (July, 1977) and Barnoski et al, "Components for Single-Strand Fiber Systems", EASCON-76 Conference Proceedings, page 119A (1976). The interaction length may be reduced to 1 cm. However, the devices are very fragile and the fabrication process is difficult to control. It is considered difficult to achieve repeatable results in different devices of this type.
An alternative means for reducing coupling length for parallel fibers is to lap polished optical flats on the sides of two fibers which are bent and fixed in pillar-shaped plastic blocks, contacting with matching oil applied at the interface of the two fibers as described in Tsujimoto et al, "Fabrication of Low-Loss 3 dB Couplers with Multimode Optical Fibers", Electronics Letters, Vol. 14, page 157 Mar. 2, 1978).
U.S. Pat. Nos. 3,933,410 of Milton and 4,087,156 of Kao et al describe special inserts or sections for a fiber bundle transmission line for coupling or mixing between pluralities of fibers. The McMahon U.S. Pat. No. 4,021,097 describes an optical coupling apparatus for coupling between a bundle or ribbon of side-by-side fibers and a specially shaped slab waveguide. This slab waveguide is extremely thin (comparable to the thickness of the optical fibers) and therefore is difficult to fabricate and is very fragile.
It has been demonstrated that when an optical fiber is bent in the form of an arc, there is an increased tendency for light to escape from the bent region in a radiation pattern which is primarily in the plane of the bend and which is directed away from the center of curvature. See for example Gambling et al, "Radiation From Curved Single-Mode Fibres", Electronics Letters, Vol. 12, page 567 (Oct. 14, 1976) and Goell et al U.S. Pat. No. 3,982,123. This effect is reciprocal, in that a greater proportion of light incident upon a fiber is coupled into the optical waveguide when the fiber is curved as compared with a straight fiber (provided the direction of incident light is consistent with the fiber orientation). Consequently, two fibers bent on a radius of curvature can be positioned such that light radiating from one fiber is coupled into the adjacent fiber with a high degree of efficiency. The degree of coupling is enhanced when the fiber cores are brought even closer together, as when optical flats are lapped and polished on the two fibers at the location where they are joined.